The room-temperature magnetic properties and magnetization reversal process of ordered (FePt)(1-x)C-x thin films, prepared by a cosputtering technique on MgO (100) substrates held at 400 degreesC substrate temperature, have been studied. Perpendicular magnetic anisotropy was obtained for the films with carbon concentrations up to 25 vol % and coercivity enhancement was observed with small carbon-doped FePt thin films. A clear spin-reorientation transition from perpendicular to the in-plane direction was observed with an increase in carbon concentration. The observed magnetic properties are intercorrelated to microstructural changes that occur as a function of the carbon doping. (C) 2003 American Institute of Physics.